高垣 直尚

Objective: This study reviewed the application of curved and bileaflet designs to pulmonary expanded polytetrafluoroethylene conduits with diameters of 10 to 16 mm and characterized this conduit on in vitro experiment, including particle image velocimetry. Methods: All patients who received this conduit between 2010 and 2022 were evaluated. Three 16-mm conduits were tested in a circulatory simulator at different cardiac outputs (1.5-3.6 L/minute) and bending angles (130°-150°). Results: Fifty consecutive patients were included. The median operative body weight was 8.4 kg (range, 2.6-12 kg); 10-, 12-, 14-, and 16-mm conduits were used in 1, 4, 6, and 39 patients, respectively. In 34 patients, the conduit was implanted in a heterotopic position. The overall survival rate was 89% at 8 years with 3 nonvalve-related deaths. There were 10 conduit replacements; 5 16-mm conduits (after 8 years) and 1 12-mm conduit (after 6 years) due to conduit stenosis, and the remaining 4 for reasons other than conduit failure. Freedom from conduit replacement was 89% and 82% at 5 and 8 years, respectively. Linear mixed-effects models with echocardiographic data implied that 16-mm conduits were durable with a peak velocity <3.5 m/second and without moderate/severe regurgitation until the patient's weight reached 25 kg. In experiments, peak transvalvular pressure gradients were 11.5 to 25.5 mm Hg, regurgitant fractions were 8.0% to 14.4%, and peak Reynolds shear stress in midsystolic phase was 29 to 318 Pa. Conclusions: Our conduits with curved and bileaflet designs have acceptable clinical durability and proven hydrodynamic profiles, which eliminate valve regurgitation and serve as a reliable bridge to subsequent conduit replacement.

Accurate estimates of momentum flux through the air-sea interface at very high wind speeds are important for predicting tropical cyclone intensities. To estimate the air-sea momentum flux under the long-fetch condition (20 m fetch) at very high wind speeds using a laboratory tank, a simple momentum flux measurement method using only four water-level gauges is conducted based on the momentum budget method. The air-water momentum flux under long-fetch conditions at very high wind speeds was measured in a typhoon simulation tank at the Research Institute of Applied Mechanics, Kyushu University. The verification was performed using previous values estimated by the eddy correlation method in a typhoon simulation tank at Kyoto University, Japan. The results showed good correlation between the values of momentum flux measured by the present momentum budget method and the other methods. The drag coefficient at very high wind speeds (41 m/s) under long-fetch conditions leveled off, as well as under the short-fetch condition (4.5 m and 6.5 m fetch). Moreover, a very weak relationship was found between the drag coefficient and the fetch. Since the maximum fetch in the present laboratory experiments is 20 m, the future field observation with the longer fetch condition will be needed for applying the results to oceans.

Effects of surface tension reduction on wind-wave growth are investigated using direct numerical simulations of air-water two-phase turbulent flows. The incompressible Navier-Stokes equations for air and water sides are solved using an arbitrary Lagrangian-Eulerian method with boundary-fitted moving grids. The wave growth of finite-amplitude and non-breaking gravity-capillary waves, whose wavelength is less than 0.07 m, is simulated for two cases of different surface tensions under a low-wind-speed condition of several metres per second. The results show that significant wave height for the smaller surface tension case increases faster than that for the larger surface tension case. Energy fluxes for gravity and capillary wave scales reveal that when the surface tension is reduced, the energy transfer from the significant gravity waves to capillary waves decreases, and the significant waves accumulate more energy supplied by wind. This results in faster wave growth for the smaller surface tension case. The effect on the scalar transfer across the air-water interface is also investigated. The results show that the scalar transfer coefficient on the water side decreases due to the surface tension reduction. The decrease is caused by suppression of turbulence in the water side. In order to support the conjecture, the surface tension effect is compared with laboratory experiments in a small wind-wave tank.

It is important to investigate the effects of current on wind waves, called the Doppler shift, at both normal and extremely high wind speeds. Three different types of wind-wave tanks along with a fan and pump are used to demonstrate wind waves and currents in laboratories at Kyoto University, Japan, Kindai University, Japan, and the Institute of Applied Physics, Russian Academy of Sciences, Russia. Profiles of the wind and current velocities and the water-level fluctuation are measured. The wave frequency, wavelength, and phase velocity of the significant waves are calculated, and the water velocities at the water surface and in the bulk of the water are also estimated by the current distribution. The study investigated 27 cases with measurements of winds, waves, and currents at wind speeds ranging from 7 to 67 ms(-1). At normal wind speeds under 30 ms(-1), wave frequency, wavelength, and phase velocity depend on wind speed and fetch. The effect of the Doppler shift is confirmed at normal wind speeds; i.e., the significant waves are accelerated by the surface current. The phase velocity can be represented as the sum of the surface current and artificial phase velocity, which is estimated by the dispersion relation of the deepwater waves. At extremely high wind speeds over 30 ms(-1), a similar Doppler shift is observed as under the conditions of normal wind speeds. This suggests that the Doppler shift is an adequate model for representing the acceleration of wind waves by current, not only for wind waves at normal wind speeds but also for those with intensive breaking at extremely high wind speeds. A weakly nonlinear model of surface waves at a shear flow is developed. It is shown that it describes dispersion properties well not only for small-amplitude waves but also strongly nonlinear and even breaking waves, which are typical for extreme wind conditions (over 30 ms(-1)).

This paper describes laboratory experiments (using a high-speed wind-wave flume) of the effects of water waves on heat and momentum exchange in the near-water atmospheric boundary layer at high wind speeds. Different from previous experiments of this type, the parameters of waves were controlled by a net stretched along the entire channel to effectively decrease the fetch. This helped to achieve dependency of the transfer coefficients on two independent parameters, namely, the wind speed and fetch (expressed via variations of the wavefield). Another key to the experiment was using a stable temperature stratification of the air flow, with the temperature of the air entering the flume 15-25 degrees higher than the water. The experiments showed a sharp increase in the heat exchange coefficient at winds exceeding 33-35 m/s, similar to that observed earlier in the high-speed wind-wave flume of Kyoto University with conditions of unstable temperature stratification of the air flow. The joint analysis of the data obtained in the high-speed wind-wave flumes of IAP RAS and Kyoto University yields the universal dependency of the exchange coefficients and the temperature roughness on the peak wave number of surface wave spectra. This is independent of the type of temperature stratification of the atmospheric boundary layer, either stable or unstable. The sharp increase in the heat exchange coefficient is shown to be associated with increased whitecapping.

気候変動の予測において大気・海洋間運動量フラックスを算出する際に用いられる抵抗係数の正確なモデル化が重要である．そこで海洋シミュレーション装置である風波水槽が有用であるが，水槽全長を超える吹走距離での海洋環境の再現が不可能である．既往研究11)では水槽出口部の波を水槽入口部で生成する波ループ法を考案することで解決した．しかし完全に吹走距離を延長するには，水槽出口部の気流も水槽入口部で生成する必要がある．本研究では，水槽出口部の風速鉛直分布を可動翼を用いて水槽入口部で生成することで水槽全長(6.5m)を超える吹走距離12m地点における気流場の再現に成功した．さらに波・気流ループを組み合わせた気流・波ハイブリッドループ法の検討した結果，ループによって波が発達する傾向が示され，吹走距離延長の可能性が示された．

Boiling heat transfer using immiscible liquid mixtures is an innovative cooling method for electronic devices operated at high heat flux density. Immiscible liquid mixtures discussed here are composed of more-volatile liquid with higher density and less-volatile liquid with lower density such as combination of FC-72 and water. In the case of pool boiling in a vessel using appropriate composed ratio of immiscible liquid mixtures, more-volatile liquid on the heating surface is boiled and as the heat flux increases, the more-volatile liquid reaches the critical heat flux. Subsequently, the less-volatile liquid is replaced with the more-volatile liquid and moves onto the heating surface, and this liquid is started boiling. This phenomenon is called a boiling refrigerant transition (BRT) and is an important feature of pool boiling by immiscible mixtures. To clear the phenomena of BRT in pool boiling by immiscible mixtures, the experiments under the conditions of various heights of more-volatile liquid layer and various mixture composed ratio by using several combinations of immiscible mixtures are carried out. And a new model derived by Kelvin-Helmholtz instability at the liquid-liquid interface is proposed for the occurrence of BRT. The unique heat transfer characteristics of the immiscible liquid mixtures were obtained from the experimental results; occurrence of BRT depends on the height of more-volatile liquid, and the characteristics after BRT is corresponding to the characteristics of pure less-volatile liquid. Experimental results including data from past literatures agreed well with newly proposed model. (C) 2019 Elsevier Ltd. All rights reserved.

Gas transfer velocities were measured in two high-speed wind-wave tanks (Kyoto University and the SUSTAIN facility, RSMAS, University of Miami) using fresh water, simulated seawater and seawater for wind speeds between 7 and 85 m s(-1). Using a mass balance technique, transfer velocities of a total of 12 trace gases were measured, with dimensionless solubilities ranging from 0.005 to 150 and Schmidt numbers between 149 and 1360. This choice of tracers enabled the separation of gas transfer across the free interface from gas transfer at closed bubble surfaces. The major effect found was a very steep increase of the gas transfer across the free water surface at wind speeds beyond 33 m s(-1). The increase is the same for fresh water, simulated seawater and seawater. Bubble-induced gas transfer played no significant role for all tracers in fresh water and for tracers with moderate solubility such as carbon dioxide and dimethyl sulfide (DMS) in seawater, while for low-solubility tracers bubble-induced gas transfer in seawater was found to be about 1.7 times larger than the transfer at the free water surface at the highest wind speed of 85 m s(-1). There are indications that the low contributions of bubbles are due to the low wave age/fetch of the wind-wave tank experiments, but further studies on the wave age dependency of gas exchange are required to resolve this issue.

The high-speed liquid-jet velocity achieved using an injector strongly depends on the piston motion, physical property of the liquid, and container shape of the injector. Herein, we investigate the liquid ejection mechanism and a technique for estimating the ejection velocity of a high-speed liquid jet using a pyro jet injector (PJI). We apply a two-dimensional numerical simulation with an axisymmetric approximation using the commercial software ANSYS/FLUENT. To gather the input data applied during the numerical simulation, the piston motion is captured with a high-speed CMOS camera, and the velocity of the piston is measured using motion tracking software. To reproduce the piston motion during the numerical simulation, the boundary-fitted coordinates and a moving boundary method are employed. In addition, we propose a fluid dynamic model (FDM) for estimating the high-speed liquid-jet ejection velocity based on the piston velocity. Using the FDM, we consider the liquid density variation but neglect the effects of the liquid viscosity on the liquid ejection. Our results indicate that the liquid-jet ejection velocity estimated by the FDM corresponds to that predicted by ANSYS/FLUENT for several different ignition-powder weights. This clearly shows that a high-speed liquid-jet ejection velocity can be estimated using the presented FDM when considering the variation in liquid density but neglecting the liquid viscosity. In addition, some characteristics of the presented PJI are observed, namely, (1) a very rapid piston displacement within 0.1 ms after a powder explosion, (2) piston vibration only when a large amount of powder is used, and (3) a pulse jet flow with a temporal pulse width of 0.1 ms.

© 2019 International Symposium on Turbulence and Shear Flow Phenomena, TSFP. All rights reserved. Effects of surface tension reduction on the momentum and scalar transfers across the wind-wave air-water interface are investigated by using a direct numerical simulation (DNS) of gas-liquid two-phase turbulent flow. The incompressible Navier-Stokes equations for both gas and liquid phases are solved using an arbitrary Lagrangian-Eulerian (ALE) method with boundary-fitted moving grids. The DNS is performed for the cases of the surface tension of water and half of it. The results show that the significant wave height for the small surface tension case becomes larger than that for the large surface tension case. The enhancement of wind wave development can be explained by increase of the form drag due to surface tension reduction. In contrast, the scalar transfer rate becomes smaller due to the surface tension reduction.

When improving wave models, it is important to determine the equilibrium-range constant for the wind-wave spectrum and find the dominant velocity parameter that well represents the equilibrium-range constant both at normal and extremely high wind speeds. Eight dominant velocity parameters were evaluated using laboratory experiments in two wind-wave tanks. One tank, installed at Kyoto University, is used for generating mechanically-generated large wind-waves. This tank has a maximum 10-m wind speed (U-10) of 67 m/s and is equipped with a programmable irregular wave generator for the loop-type wave-generation method, which was originally developed in our previous study. The other tank, installed at Kyusyu University, is used for generating pure large wind-waves, and has a total length of 54 m. The results show that at extremely high wind speeds the equilibrium-range constants for mechanically-generated large wind-waves agree with those of pure small wind-waves. In addition, comparison of the correlation coefficients of the eight dominant velocity parameters with the equilibrium-range constant reveals that three dominant velocity parameters of (u*C-2(p))(1/3), ((U10Cp)-C-2)(1/3), and U-LS/2 (u*: friction velocity; C-p: phase velocity; U-LS/2: wind velocity at half the height of wavelength over the ocean) well correlate with the equilibrium-range constant at both normal and extremely high wind speeds.

Heat and momentum transfer across the wind-driven breaking air-water interface at extremely high wind speeds was experimentally investigated using a high-speed wind-wave tank. An original multi-heat-balance method was utilized to directly measure latent and sensible heat transfer coefficients. The results show that both heat transfer coefficients level off at low and normal wind speeds but increase sharply at extremely high wind speeds. The coefficients have a similar shape for wind speeds at a height of 10 m. Therefore, the wind speed dependence on the latent and sensible heat transfer coefficients can be represented by that of the enthalpy coefficient even in the extremely high-speed region. To show how significantly the drag and enthalpy coefficients affect the intensity of tropical cyclones, the coefficients were applied to Emanuel's analytic model. The analytic model shows that the difference between the present laboratory and conventional correlations significantly affects the maximum storm intensity predictions, and the present laboratory enthalpy and drag coefficients have the remarkable effect on intensity promotion at extremely high wind speeds. In addition, the simulations of strong tropical cyclones using the Weather Research and Forecasting (WRF) Model with the present and conventional correlations are shown for reference in the appendix. The results obtained from the models suggest that it is of great importance to propose more reliable correlations, verified not only by laboratory but also by field experiments at extremely high wind speeds.

Accurate measurements of particle number density along with particle diameters and velocities are strongly required both in academic and industrial fields. A new imaging technique, through the evaluation of the effective depth of field of a camera, is developed using standard solid particles with constant diameters. To measure the effective depth of field for a wide range of particle diameters, three optical setups, named microscale, mesoscale, and macroscale setups, are used for the diameters of 50 mu m - 201 mu m, 201 mu m - 3.97 mm, and 3.97 mm - 15 mm, respectively. The measured effective depth of field is further applied to measure the size dependence of the number density of entrained bubbles by breaking waves in a wind-wave tank. The results show that the slopes of the number density of the entrained bubbles in the experiments corresponded to those measured by a phase Doppler particle analyzer under 500 mu m, and was -5 over 500 mu m in both fresh and salt waters. This emphasizes that the present imaging technique can measure the diameters and particle number density with high precision and is important for measurements of droplets, bubbles, and solid particles with a wide range of diameters.

It is important to develop a wave generation method for extending the fetch in laboratory experiments, because current laboratory studies are limited to fetch shorter than 100 m. Two wave generation methods are proposed for generating wind waves under long fetch conditions in a wind-wave tank using a programmable irregular-wave generator. The first method is the spectral-model-based wave-generation method (SBWGM), which is appropriate at normal wind speeds for extending the fetch. The SBWGM also can be used at extremely high wind speeds if we know the spectral shape. In SBWGM, a conventional model of the wind-wave spectrum is used for the movement of the programmable irregular-wave generator. The second method is the loop-type wave-generation method (LTWGM), which can be used at wide range of wind speeds and is especially appropriate to be used at extremely high wind speeds, where the spectral shape is unknown. In LTWGM, the waves whose characteristics are most similar to the wind waves measured at the end of the tank are reproduced at the entrance of the tank by the programmable irregular-wave generator to extend the fetch. Water-level fluctuations are measured at both normal and extremely high wind speeds using resistance-type wave gauges. The results show that SBWGM can produces wind waves with a fetch over 500 m, but only at normal wind speeds. However, LTWGM can produce wind waves with long fetches exceeding the length of the wind-wave tank across a broad range of wind speeds, but considerable time is required to produce wind waves at long-fetch conditions, i.e. fetch over 500 m. It is observed that the wind-wave spectrum with a long fetch reproduced by SBWGM is consistent with that of the modelled wind-wave spectrum, although the generated wind waves are different from those in the open ocean because of the finite width of the tank. In addition, the fetch laws with significant wave height and period are confirmed for wind waves under long-fetch conditions. This implies that the ideal wind waves under long-fetch conditions can be reproduced using SBWGM with the programmable irregular-wave generator. (C) 2018 The Authors. Published by Elsevier B.V.

It is important to develop a wave-generation method for extending the fetch in laboratory experiments, because previous laboratory studies were limited to the fetch shorter than several dozen meters. Anew wave-generation method is proposed for generating wind waves under long-fetch conditions in a wind-wave tank, using a programmable irregular-wave generator. This new method is named a loop-type wave-generation method (LTWGM), because the waves with wave characteristics close to the wind waves measured at the end of the tank are reproduced at the entrance of the tank by the programmable irregular-wave generator and the mechanical wave generation is repeated at the entrance in order to increase the fetch. Water-level fluctuation is measured at both normal and extremely high wind speeds using resistance-type wave gauges. The results show that, at both wind speeds, LTWGM can produce wind waves with long fetches exceeding the length of the wind-wave tank. It is observed that the spectrum of wind waves with a long fetch reproduced by a wave generator is consistent with that of pure wind-driven waves without a wave generator. The fetch laws between the significant wave height and the peak frequency are also confirmed for the wind waves under long-fetch conditions. This implies that the ideal wind waves under long-fetch conditions can be reproduced using LTWGM with the programmable irregular-wave generator.

<p>The drag coefficient is very important parameter that is used to calculate the air-sea momentum flux. Although the many estimation models of the drag coefficient have been proposed and discussed (conditions with extreme wind, wind wave, swell, etc.), the agreement has not reached yet. Comparison and evaluation of the model's characteristics have been performed. However, assessment related climate change has not been done. For a global phenomenon associated with climate change due to change in wind speed, we focused on the El Nino and La Nina phenomenon. In this study, using the sea surface wind data, we investigated the influence of the air-sea momentum flux estimation for the year when the phenomenon occurred and did not occur. CCMP (Cross-Calibrated Multi-Platform) of NASA was used the sea surface wind data. The period is from January to December of El Nino phenomenon, La Nina phenomenon, and normal period. Studies of Charnock (1955) and Takagaki et al. (2012) (consider the extreme wind speed range) were used for the drag coefficient model. The annual mean global air-sea momentum flux showed that the maximum and minimum air-sea momentum flux value corresponded the normal period and the El Nino phenomenon, respectively. And the difference was as small as 7.4% and 7.0% in both models. In every month, it showed the maximum and minimum is the normal period and the El Nino phenomenon in both models, and the difference was as big as approximately 13.9% and 13.8%. We also investigated the difference of the air-sea momentum flux for each phenomena in 10 degree latitude bands and the proposed seven sea areas. As a results, the difference between the maximum and the minimum value corresponded to the La Nina and El Nino phenomenon showed approximately 18% from the north latitude 50° to the north latitude 60° in the high wind speed region. The wind speed in the North Atlntic showed that sea wind speed is a very large value for the El Nino and La Nina phenomenon. Therefore, in this region, the value of air-sea momentum flux corresponding to the El Nino and La Nina phenomenon was larger than that corresponding to the other phenomena.</p>

The mass transfer across a sheared gas-liquid interface strongly depends on the Schmidt number. Here we investigate the relationship between mass transfer coefficient on the liquid side, k(L), and Schmidt number, Sc, in the wide range of 0.7 <= Sc <= 1000. We apply a three-dimensional semi direct numerical simulation (SEMI-DNS), in which the mass transfer is solved based on an approximated deconvolution model (ADM) scheme, to wind-driven turbulence with mass transfer across a sheared wind-driven wavy gas-liquid interface. In order to capture the deforming gas-liquid interface, an arbitrary Lagrangian-Eulerian (ALE) method is employed. Our results show that similar to the case for flat gas-liquid interfaces, k(L) for the wind-driven wavy gas-liquid interface is generally proportional to Sc-0.5, and can be roughly estimated by the surface divergence model. This trend is endorsed by the fact that the mass transfer across the gas-liquid interface is controlled mainly by streamwise vortices on the liquid side even for the wind-driven turbulence under the conditions of low wind velocities without wave breaking.

Turbulent heat transfer across a sheared wind-driven gas-liquid interface is investigated by means of a direct numerical simulation of gas-liquid two-phase turbulent flows under non-breaking wave conditions. The wind-driven wavy gas-liquid interface is captured using the arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids, and the temperature fields on both the gas and liquid sides, and the humidity field on the gas side are solved. The results show that although the distributions of the total, latent, sensible and radiative heat fluxes at the gas-liquid interface exhibit streak features such that low-heat-flux regions correspond to both low-streamwise-velocity regions on the gas side and high-streamwise-velocity regions on the liquid side, the similarity between the heat-flux streak and velocity streak on the gas side is more significant than that on the liquid side. This means that, under the condition of a fully developed wind-driven turbulent field on both the gas and liquid sides, the heat transfer across the sheared wind-driven gas-liquid interface is strongly affected by the turbulent eddies on the gas side, rather than by the turbulent eddies and Langmuir circulations on the liquid side. This trend is quite different from that of the mass transfer (i.e. CO2 gas). This is because the resistance to heat transfer is normally lower than the resistance to mass transfer on the liquid side, and therefore the heat transfer is controlled by the turbulent eddies on the gas side. It is also verified that the predicted total heat, latent heat, sensible heat and enthalpy transfer coefficients agree well with previously measured values in both laboratory and field experiments. To estimate the heat transfer coefficients on both the gas and liquid sides, the surface divergence could be a useful parameter, even when Langmuir circulations exist.

Previous studies have demonstrated the saturation of drag coefficients at strong wind speeds. But the mechanism behind this saturation has not yet been fully clarified. In this study, at normal and strong wind speeds, we use a wind-wave tank for investigating the peak enhancement factor of the wind-sea spectrum, which is an appropriate wave parameter for representing interfacial flatness. We measured the water-level fluctuation using wave gauges. At strong wind speeds, the result shows that the peak enhancement factor of the wind-sea spectrum decreases with decreasing inverse wave age and with increasing wind speed. This suggests that the distinctive wind-wave breaking occurs at strong wind speeds. It also suggests that this distinctive breaking of wind waves causes the saturation of drag coefficients at strong wind speeds.

© Published under licence by IOP Publishing Ltd. The equilibrium range of wind-waves at normal and extremely high wind speeds was investigated experimentally using a high-speed wind-wave tank together with field measurements at normal wind speeds. Water level fluctuations at normal and extremely high wind speeds were measured with resistance-type wave gauges, and the wind-wave spectrum and significant phase velocity were calculated. The equilibrium range constant was estimated from the wind-wave spectrum and showed the strong relationship with inverse wave age at normal and extremely high wind speeds. Using the strong relation between the equilibrium range constant and inverse wave age, a new method for estimating the wind speed at 10-m height (U 10) and friction velocity (u∗) was proposed. The results suggest that U 10 and u∗ can be estimated from wave measurements alone at extremely high wind speeds in oceans under tropical cyclones.

Drag coefficient is an important parameter when estimating the air-sea momentum flux correctly. The drag coefficient, however, hasn't been accurately established due to variations in the data from field observation. Thus, a number of drag coefficient models have been formulated. Since these models define an effective low wind speed range (e.g., 6 m/s), it is important to correctly estimate the air-sea momentum flux in such an effective low wind speed range. Nevertheless, with regard to such models, the air-sea momentum flux is commonly extrapolated out of the effective low wind speed range that is defined for each model. Therefore, such an estimated drag coefficient is not always correct, and the difference in the drag coefficient is reflected by the particular model that is used. In this study, we investigated the effect of the various drag coefficient models concerning the air-sea momentum for the low wind speed range in two processes: (1) calculating the drag coefficient in the effective low wind speed range, and (2) extrapolating the drag coefficient out of the range. Six drag coefficient models were used (Charnock, 1955; Smith, 1980; Large and Pond, 1981; Yelland and Taylor, 1996; Large and Yeager, 2004; Takagaki et al., 2012). We found the largest difference between the maximum and the minimum annual mean global air-sea momentum flux on the estimated data in the effective low wind speed range at 98.5% while 19.1% was observed on the extrapolated data. When taking into consideration both the 10-degree latitude and the proposed seven sea areas, we also found that significant impact on the air-sea momentum flux was apparent when the occurrence frequency of low wind speed was high. These results show that the parametrization of the drag coefficient is imperative for the low wind speed range.

Air-sea CO₂ gas transfer velocity is used to estimate the air-sea CO₂ gas flux and is generally expressed as a function of wind speed. There has been considerable research on the air-sea CO₂ gas transfer velocity including wind speeds due to the fact that the difference in wind speeds impacts the air-sea CO₂ gas flux. On the other hand, CO₂ solubility to estimate the air-sea CO₂ gas flux and the Schmidt number to measure the air-sea CO₂ gas transfer velocity are expressed as a function of sea surface temperature. Given this, different data sets of global sea surface temperature have been proposed. Therefore, it is imperative to evaluate the effect of the air-sea CO₂ gas flux caused by the difference in sea surface temperature. In this study, we estimated and then investigated the global air-sea CO₂ gas flux using wind and wave data sets by ECMWF, as well as seven kinds of sea surface temperature data sets (ERA40, JRA-25, JRA-55, NCEP R1, NCEP R2, ERA-interim-high, and ERA-interim-low). Our findings show that the largest difference of the data sets in annual global air-sea CO₂ gas flux was 13.2%, and the largest difference by 10-degree latitude was 0.11 (PgC/year) at 60–70 degrees south latitude. To conclude, these results clearly demonstrate that the difference in sea surface temperature has a significant effect on the air-sea CO₂ gas flux.

We propose a new model with microbreaking and linear dependence on wind speed in the whitecap-free area and linear dependence on the fractional area of whitecaps, based on the whitecap model proposed by Monahan and Spillane (The role of oceanic whitecap in air-sea gas exchange. In: Brutsaert W, Jirka GH (eds) Gas transfer at water surfaces. Reidel, Dordrecht, 495-503, 1984). The relationship between whitecap coverage and wind-sea Reynolds number proposed by Zhao and Toba (J Oceanogr 57:603-616, 2001) provides the link among waves, wind, and whitecap coverage. This model yields a mass transfer velocity that is linear in wind speed at light winds and smoothly transitions with increasing wind to approximately cubic in wind speed and linear in wave age.

The effects of turbulent eddies and Langmuir circulations in liquid flow on scalar transfer across a sheared wind-driven gas-liquid interface are investigated by means of a direct numerical simulation of a gas-liquid two-phase turbulent flow with a wind-driven nonbreaking wavy interface. The wind-driven wavy gas-liquid interface is captured using an arbitrary Lagrangian-Eulerian method with boundary-fitted coordinates on moving grids. The results show that Langmuir circulations are generated on the liquid side below the sheared wind-driven gas-liquid interface. The marker particles on the gas-liquid interface, the turbulent eddies in the form of streamwise vortices on the liquid side (i.e., the typical horseshoe vortices associated with bursting motions), and the low scalar flux lines on the gas-liquid interface induced by the turbulent eddies on the liquid side tend to locally concentrate in the regions along the downward flows caused by the Langmuir circulations. It is suggested that the turbulent eddies on the liquid side mainly control the scalar transfer across the sheared wind-driven gas-liquid interface, and the effect of the Langmuir circulations is relatively small. (C) 2015 AIP Publishing LLC.

The air-sea CO₂ gas transfer velocity is generally expressed as a function only of the wind speed U₁₀. However, there exists considerable disagreement among the observed values. The disagreement is especially large in the context of the different sea surface conditions (wind wave growth and swell etc.). Consequently, many models of the air-sea CO₂ gas transfer velocity are proposed by field and laboratory experiments. In this study, we evaluate the estimated global air-sea CO₂ gas flux using the different some air-sea CO₂ gas transfer velocity models (field experiment model, laboratory experiment model and hybrid model considering wave breaking). The 6-hourly wind speed and mean period of wind and wave data sets by ECMWF were used. The maximum difference of annual global air-sea CO₂ gas flux was 0.76 PgC/year. The annual global air-sea CO₂ gas flux of each laboratory experiment models were the smallest value, and each hybrid models were near value to each field experiment models. The difference of each model in low latitude is large, same as the difference in middle latitude. This shows that the difference of the result of each model in low latitude is significant for the estimation of air-sea CO₂ gas flux.

Drag coefficient is an important parameter in order to correctly estimate the air-sea momentum flux. However, the parametrization of the drag coefficient hasn't been established due to the variation in the field data. Instead, a number of drag coefficient model formulae have been proposed, even though almost all these models haven't discussed the extreme wind speed range. With regards to such models, it is unclear how the drag coefficient changes in the extreme wind speed range as the wind speed increased. In this study, we investigated the effect of the drag coefficient models concerning the air-sea momentum flux in the extreme wind range on a global scale, comparing the difference in the drag coefficient models between Charnock (1955) and Takagaki et al. (2012). Interestingly, the former model didn't discuss the extreme wind speed range while the latter one considered it. We have found that the difference of the models in the annual global air-sea momentum flux was small because the occurrence frequency of strong wind was approximately 1% with a wind speed of 20 m/s or more. However, we also discovered that the difference of the models was shown in the middle latitude where the annual mean air-sea momentum flux was large. In addition, the estimated data showed that the difference of the models in the drag coefficient was large in the extreme wind speed range and that the largest difference became 23% with a wind speed of 35 m/s or more. These results clearly show that the difference of the two models concerning the drag coefficient has a significant impact on the estimation of a regional air-sea momentum flux in an extreme wind speed range such as that seen in a tropical cyclone environment.

The characteristics of vortex rings induced below the free water surface due to the impingement of a falling single drop with small drop diameters and two impact angles were investigated by means of a three-dimensional direct numerical simulation (DNS) with a Level-Set method. The results show that the present DNS well captures the structures and transport processes of a crater vortex-ring (CV) and a water-column vortex-ring (WCV) due to the vertical impingement of a drop, and that the intensity of the WCV is larger than that of the CV. In the oblique impingement of a drop, a CV is similarly generated, but a vortex tube (VT) creeping along the free surface is also generated instead of the WCV, and the intensity of the VT is much smaller than that of the CV. The relationship between the momentum of a drop and the intensity of the dominant vortex ring is determined independently of diameter and impact angle and is also consistent with the previous measurements (Takagaki and Komori, 2014). (C) 2014 Elsevier Ltd. All rights reserved.

The drag coefficient C-D is generally expressed as a function only of the wind speed U-10. However, there exists considerable disagreement among the observed values of C-D. In this study, we investigate the variation of C-D by using data sets, including directional wave spectral data in long period measurements. Wind and wave data obtained at a coastal tower operated by the Shirahama Oceanographic Observatory of the Disaster Prevention Research Institute (DPRI) at Kyoto University in Japan, together with data sets obtained by Suzuki et al. (2002), were used. The directional wave spectra were separated into four different groups: the following and cross swell cases, mixed swell case, and pure wind-wave case. The data sets also showed variation in wind speed regions higher than 20 m/s in the C-D-U-10 diagram. It was shown that this variation is the effect of the natural fluctuation of wind. We also observed C-D values by using the windsea Reynolds number R-B proposed by Toba et al. (2006). In the following swell cases, C-D values are slightly smaller. In the cross swell cases, when the difference in direction between the wind and waves is greater than 70 degrees, C-D has larger values. However, C-D has similar values in the following swell cases when the difference in direction between the wind and waves is less than 70 degrees.

The mass transfer mechanism across the air-water interface due to the impingement of a single liquid drop was investigated through laboratory experiments using particle imaging velocimetry (PIV) and planar laser-induced fluorescence technique (PLIF). Velocity and CO2 concentration fields in the liquid after the impingement were visualized. The results show that the impingement of a single liquid drop on the water surface generates several vortex rings near the water surface. The vortex rings renew the water surface and also convect the CO2 gas dissolved near the water surface downward. The vortical motion clearly shows that the vortex rings work as surface-renewal eddies. The radius, center velocity and presence time of surface-renewal eddies increase with increasing momentum of the impinging drop. This suggests that surface-renewal eddies with larger radius and faster center velocity are induced by the impingement of a single drop with larger vertical momentum, and air-water mass transfer is promoted by such eddies. Based on the surface-renewal concept including the area and time fractions, a model for the air-water mass transfer due to multiple impingements of drops is also proposed. (C) 2013 Elsevier Ltd. All rights reserved.

© 2013 K. Iwano et al. Mass transfer velocity kL across the wind-driven air-water interface was estimated at extremely high wind speeds (up to U10=70ms-1) in a high-speed wind-wave tank by measuring changes in CO2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. kL increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of kL changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. kL is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present kL agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999) and Wanninkhof et al. (2009) for low and moderate freestream wind speeds. However, for extremely high free-stream wind speeds, the present data deviate upward from the correlation curves of Wanninkhof (1992) and Wanninkhof and McGillis (1999) and approach to that of Wanninkhof et al. (2009) as the wind speed increases. This indicates that the correlation curve of Wanninkhof et al. (2009) is more appropriate for the correlation between kL and free-stream wind speed than those of Wanninkhof (1992) and Wanninkhof and McGillis (1999) in extremely high wind speed region.

Mass transfer velocity k(L) across the wind-driven air-water interface was estimated at extremely high wind speeds (up to U-10 = 70 m s(-1)) in a high-speed wind-wave tank by measuring changes in CO2 concentration in the water. In addition, the volume flux of dispersing droplets lost from the tank and the wave height were measured. k(L) increases drastically with wind speed at extremely high wind speeds. The volume flux of dispersing droplets begins to increase drastically and the mean height of significant waves changes its rate of increase at almost the same wind speed as that at which the rate of increase of k(L) changed. These results suggest that intense wave breaking occurs at extremely high wind speeds and it has significant effects on mass transfer. k(L) is well correlated with the free-stream wind speed for both present laboratory and previous field measurements in the low and moderate wind speed regions. Present k(L) agrees well with the conventional correlation curves proposed by Wanninkhof (1992), Wanninkhof and McGillis (1999) and Wanninkhof et al. (2009) for low and moderate free-stream wind speeds. However, for extremely high free-stream wind speeds, the present data deviate upward from the correlation curves of Wanninkhof (1992) and Wanninkhof and McGillis (1999) and approach to that of Wanninkhof et al. (2009) as the wind speed increases. This indicates that the correlation curve of Wanninkhof et al. (2009) is more appropriate for the correlation between k(L) and free-stream wind speed than those of Wanninkhof (1992) and Wanninkhof and McGillis (1999) in extremely high wind speed region.

Momentum transfer across the wind-driven breaking air-water interface under strong wind conditions was experimentally investigated using a high-speed wind-wave tank together with field measurements at normal wind speeds. An eddy correlation method was utilized to measure roughness length and drag coefficient from wind velocity components measured by laser Doppler and phase Doppler anemometers. As a result, a new model for the roughness length and drag coefficient was proposed for predicting momentum transfer across the sea surface under both normal and strong wind conditions using the universal relationship between energy and significant frequency of wind waves normalized by the roughness length. The model shows that the roughness length and drag coefficient are uniquely determined at all wind speeds by energy and significant frequency of wind waves, and they can be given against U-10 only from the measurements of the wave parameters and one-point mean air velocity in the logarithmic law region. Citation: Takagaki, N., S. Komori, N. Suzuki, K. Iwano, T. Kuramoto, S. Shimada, R. Kurose, and K. Takahashi (2012), Strong correlation between the drag coefficient and the shape of the wind sea spectrum over a broad range of wind speeds, Geophys. Res. Lett., 39, L23604, doi:10.1029/2012GL053988.

The subgrid scale (SGS) variance for a high-Schmidt-number passive scalar of Sc >> 1 is measured using a high-resolution planar laser-induced fluorescence technique in a grid-generated turbulent liquid flow, and the values of the model coefficients in the scale-similarity model and the scalar-gradient model used for estimating the SGS scalar variance are experimentally evaluated. The results show that for both models, the measured values are much larger than the well-known values obtained in the previous studies done for non-high-Sc scalars of Sc congruent to 1. Similarly, the measured value of the model coefficient in the scalar-gradient model tends to be larger than the value estimated by the dynamic procedure. The increases in the measured values of the model coefficients for the high-Sc scalar can be explained by the presence of the viscous-convective range showing a nearly (-1)-slope in the high-wavenumber range of the power spectrum of concentration fluctuation. (C) 2011 American Institute of Chemical Engineers AIChE J, 58: 377-384, 2012

A three-dimensional direct numerical simulation (DNS) with Level-Set method is applied to the impingement process of a single droplet with diameter of 1.2〜2.2mm on the air-water free surface, and the characteristics of vortices induced below the free surface are investigated by comparing with the experiments. The results show that the present DNS can capture the generation and transportation process of the vortices. The predicted vortex intensity corresponds with the experimental bestfit curve proposed for a larger droplet with diameter of 2.2〜5.6mm. In addition, the comparisons of velocity vectors and scalar concentration fields between the vertical and oblique impingements show that the momentum and mass transfer mechanism are strongly affected by the angle of droplet's impingement.

The mechanism of mass transfer due to a single droplet impinging on the air-water interface was investigated using PIV and LIF techniques. Velocity and CO₂ concentration fields in the liquid side after the impingement of a single droplet were visualized. The radius and the velocity of the surfacerenewal vortex induced by the droplet impingement were estimated. It is found that CO₂ near the free surface goes downward due to the surface-renewal vortex induced by the impinging droplet, and both the radius and velocity of the vortex increase with increasing the vertical momentum of the droplet. This suggests that the droplet impinging on the free surface promotes mass transfer across the air-water interface.

[1] The effects of rainfall on mass transfer across the air-water interface and turbulent structure were investigated through laboratory experiments in a turbulent open-channel flow. The air-water CO2 transfer coefficient due to raindrops impinging on the air-water surface and turbulence quantities in an open-channel flow were measured. The results show that rainfall enhances turbulent mixing near the free surface on the liquid side, and the enhancement results in increase of CO2 transfer across the air-water interface. The mass transfer coefficient on the liquid side is well correlated with the mean vertical momentum flux of rainfall (MF), but it is not well correlated with the mean kinetic energy flux of raindrops impinging on the unit area of the air-water interface (KEF), as has been proposed in previous studies. The maximum value of the mass transfer coefficient obtained in the present rain experiments corresponds to that observed in oceans with high wind speeds. This suggests that it is of great importance to consider the effects of rainfall in precisely estimating the local air-sea CO2 exchange rate.

The effects of impinging raindrops on both turbulence below the air-water interface and CO2 transfer across the air-water interface are discussed using laboratory measurements by Takagaki and Komori [1]. The measurements of CO2 absorption rate and turbulence quantities in an open-channel flow show that impinging raindrops enhance both turbulent mixing near the free surface on the liquid side and CO2 transfer across the air-water interface, and that the mass transfer velocity due to impinging raindrops is well correlated with the mean vertical momentum flux of raindrops. The reason why the mass transfer velocity is well correlated by the mean vertical momentum flux is explained by showing the instantaneous velocity vectors induced by a falling single droplet. Further, in order to clarify the effects of rainfall on the global and local CO2 transfer across the air-sea interface, the mean annual net air-sea CO2 flux was estimated using both the daily precipitation data set and the empirical correlation [1] between the mass transfer velocity and mean vertical momentum flux. The rainfall effects are also compared with wind shear effects. The results show that rainfall effects are significant for the local CO2 budget between atmosphere and ocean in equatorial and mid-latitude regions, but are not so important for global budget, compared to the wind shear effect.

The effects of rainfall on both mass transfer across the air-water interface and turbulence structure in the interfacial region were investigated through laboratory experiments in a turbulent open-channel flow. The CO₂ absorption rate by rain droplets impinging on the free surface and turbulence quantities were measured. The results show that the rainfall enhances the turbulent mixing near the free surface on the liquid side and the CO₂ transfer across the air-water interface. The mass transfer coefficient on the liquid side is well correlated by the mean kinetic energy of rain droplets impinging on the unit area of the air-water interface, KEF, and it is proportional to the square root of KEF. The maximum value of the mass transfer coefficient obtained in this study corresponds to that obtained in wind-driven turbulence with wind speed 15 m/s. This suggests that it is of great importance to consider the effects of rainfall on the CO₂ exchange rate in a general circulation model for estimating the global warming.